Optical displacement sensors are being developed for use as optical microphones, and can provide advantages over the conventional microphones in terms of increased sensitivity, increased frequency range, and reduced electronic noise. Previous optical displacement sensors have generally operated passively by measuring light directed through a Fabry-Perot interferometer, or from a reflective diffraction grating.
The present invention operates differently from previous optical displacement sensors in utilizing optical feedback into a vertical-cavity surface-emitting laser (VCSEL) from an optical cavity formed by a moveable membrane and an output mirror of the VCSEL. This results in the optical cavity of the VCSEL being coupled to the optical cavity formed by the moveable membrane and output mirror of the VCSEL. As a result, any movement of the membrane in response to sound, vibration, acceleration, etc., will produce a change in the lasing characteristics of the VCSEL, and in particular can change the intensity of a beam of lasing light produced by the VCSEL.
The present invention can reduce the effect of squeezed-film damping which is present in other types of optical displacement sensors which require a membrane to be located very close to an adjacent substrate. In the present invention, the membrane can be suspended above the VCSEL with a predetermined separation which can be as large as several millimeters. This greatly reduces any squeezed-film damping.
Many different embodiments of the optical displacement sensor of the present invention are possible which can be batch fabricated as single devices or as an array of devices.
These and other advantages of the present invention will become evident to those skilled in the art.